\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{patterns}
\begin{document}
\footnotesize
  \begin{tikzpicture}[>=stealth,scale=0.7]
    \draw[white](0,3.2)--++(1,0);
    \draw[thick](0,0)--++(0,2.2)arc(180:90:0.8)--++(3,0)arc(90:0:1.2)--++(0,-1.8)--cycle;
    \foreach \x in {0.8,1.8,2.8} 
      {
        \draw[thick] (\x,2.2)circle(0.3);
        \draw[thick] (\x,2.2)circle(0.4);
      }
    \draw[thick](7,0)rectangle(7.2,3);
    \draw[thick](6.75,1.8)rectangle(6.95,2.6);
    \draw[thick](7.2,1.8)rectangle(7.4,2.6);
    \draw[thin](6.95,1.7)--++(0,-1);
    \draw[thin,->](6.5,1)--(6.95,1)node[at start,above right]{$f$};
    \draw[thin,->](7.3,1)--(7,1);
  \end{tikzpicture}
\end{document}